Persistence of depositional features in a strongly seasonal, tide-dominated delta

The Ganges-Brahmaputra-Meghna (GBM) Delta is one of the largest deltas in the world, covering more than 100,000 km2. The GBM River system transports over one billion tons of sediment annually and delivers ~750 million tons to the Bay of Bengal, ~25% of which is advected by tides into the fluvially abandoned western delta plain. Fluvial sediment and water discharge are controlled by the southwest Asian monsoon, with wet season discharge rates exceeding those of the dry season by a factor of five. The Sundarbans National Forest (SNF) is located within the GBM Delta and, covering more than 10,000 km2, is the world's largest continuous mangrove stand. The present rate of sediment delivery allows the SNF platform elevation to keep pace with regional sea level rise, but India's National River Linking Project (NRLP) could decrease the suspended sediment loads of Ganges and Brahmaputra Rivers by 39-75% and 9-25% respectively, and thus may change future sedimentation. In this study, we examine whether sediment stored in the tidal channels could serve as an additional source for platform nourishment, if resuspended and delivered to the mangrove platform throughout the year. Over two field seasons, 70 shallow (<50 cm) sediment cores were collected from the channel banks, imaged using xradiography to observe fine scale changes in depositional characteristics on tidal channel banks, and subsampled for textural and geochemical analyses. In more than half of the cores examined there is no change in the style of the laminations, suggesting that the processes controlling deposition remain constant throughout the year, despite the seasonal change in the external sediment supply. Cores where the depth of laminations decreases from wet to dry season demonstrate that sediment is being eroded from the banks and advected elsewhere, with the mangrove platform a likely sink. Over 70% of cores showed surface (0-2 cm) grain sizes coarsening from the wet to the dry season, which could be the product of the winnowing of fine sediments as overall sediment supply decreases and material with lower bulk density is preferentially taken away. The findings from textural analysis were further supported by total organic carbon (TOC) profiles, which also indicate constant reworking of sediment on the channel banks throughout the year.

Automated summary

The Ganges-Brahmaputra-Meghna Delta is one of the largest deltas in the world and transports a large amount of sediment. This study investigates whether sediment stored in tidal channels could serve as an additional source for platform nourishment. The results suggest that the sedimentation process remains relatively constant throughout the year, and the suspended sediment loads could be affected by India's National River Linking Project.